1. Field of the Invention
This invention relates to improved slag-based cementing composition and to improved methods of using such compositions for cementing oil and gas wells.
2. Description of the Prior Art
During construction of oil and gas wells, a rotary drill is typically used to bore through subterranean formations of the earth to form a borehole. As the rotary drill bores through the earth, a drilling fluid, known in the industry as a "mud," is circulated through the borehole. Drilling fluids are usually pumped from the surface through the interior of the drill pipe. By continuously pumping the drilling fluid through the drill pipe, the drilling fluid can be circulated out the bottom of the drill pipe and back up to the well surface through the annular space between the wall of the well bore and the drill pipe. The hydrostatic pressure created by the column of mud in the hole prevents blowouts which would otherwise occur due to the high pressures encountered within the well. The drilling fluid is also used to help lubricate and cool the drill bit and facilitates the removal of cuttings as the borehole is drilled.
Once the well bore has been drilled, casing is lowered into the well bore. A cement slurry is then pumped into the casing and a plug of fluid, such as drilling mud or water, is then pumped behind the cement slurry in order to force the cement up into the annulus between the exterior of the casing and the borehole. The cement slurry is then allowed to set and harden to hold the casing in place. The cement also provides zonal isolation of the subsurface formations, helps to prevent sloughing or erosion of the well bore and protects the well casing from corrosion from fluids which exist within the well.
Typically, hydraulic cements, particularly Portland cements, are used to cement the well casing within the well bore. Hydraulic cements are cements which set and develop compressive strength due to the occurrence of a hydration reaction which allows them to set or cure under water. The physical properties of the set hydraulic cement relate to the crystalline structure of the calcium-silicate-hydrates formed during hydration. Conventional Portland cements form an interlocking crystalline network of, for example, tricalcium silicate, dicalcium silicate, tetracalcium aluminum ferrite and calcium hydroxide crystals. These crystals interconnect to form an interlocking crystalline structure which provides both flexural strength and a degree of resiliency.
In recent years, there has been growing emphasis on the conversion of drilling fluids to cementitious slurries so that the converted drilling fluid can be pumped into the casing and used to cement the casing within the well bore. One characteristic of such drilling fluids which are convertible "in situ" to a cement is the addition of blast furnace slag which initiates the setting or hardening of the converted drilling fluid.
Blast furnace slags, which are produced as a by-product during the smelting process of metal ores, have been used in the past for cementing purposes in the oil and gas well industries. The use of these slags is an economical means of converting the drilling fluid to a cementitious composition. Set slag cements, however, have less flexural strength and are more brittle than their Portland cement counterparts. This brittleness may result in excessive damage of the cement during perforating operations and may also reduce the structural integrity of the cement surrounding the casing.